Thermoluminescent radiation pellets

ABSTRACT

1. A PROCESS FOR PREPARING PELLETS ADAPTED TO BE MADE THERMOLUMINESCENT UPON ESXPOSURE TO RADIATION COMPRISING THE STEP EITHER OF PRESSING OR EXTRUDING A POWDER OF A THERMOLUMINESCABLE MATERIAL AT A PRESSURE OF AT LEAST 10,000 P.S.I. WHEN AT A TEMPERATURE OF AT LEAST ROOM TEMPERATURE AND NO HIGHER THAN 825*C., SAID PRESSURE BEING (AT LEAST 400,000 P.S.I. WHEN SAID TEMPERATURE IS IN THE RANGE OF ROOM TEMPEATURE TO 100*C., AT LEAST 300,000 P.S.I. WHEN SAID TEMPERATURE IS IN THE RANGE OF 100* TO 200*C., AND AT LEAST 200,000 P.S.I. WHEN IN THE RANGE OF 200-300*C.) LESS THAN 1.5 MILLION P.S.I., GREATER PRESSURES BEING REQUIRED AT LOWER TEMPERATURES.

151-75 OR EZEE O Feb. 18, 1975 c, STEWART ETAL Re. 28,340

THERMOLUIINISCINT RADIATION PEI-LETS Original Filed Aug. 22, 1966 2Sheets-Shut 1 INVENTORS ELMER GSTEWART WILLIAM S. FOULKS. JR. MdlmghM ATTOR NEY Feb. 18, 1915 E, TE ART n AL 11.. 25,340

THEHMOLUIINISGINT RADIATION PELLETS Original Filed Aug 22, 1966 2Shuts-Shoot 2 FIG. 3 F164 |.5oo.ooo

PRESSURE (25.1.) vs. TEMPERATURE Ge) Loo FOR exmuume AND omEcT PRESSINGo|= THERMOLUMWESCABLE 5 500.000 LITHIUM FLUORlDE d a 300,000 U) D Z 8o5o,oooran EXTRUSION Q. l a: FOR PRESS/N6 3 10,000 m U7 5 40.000 0.

IOOOO i r 4 TEMPERATURE DEGREES C FIGS INVENTORS ELMER C. STEWARTWILLIAM S. F O ULKS. JR.

U ATTORNEY United States Patent Re. 28,340 Reissued Feb. 18, 1975 28,340THERMOLUMINESCENT RADIATION PELLETS Elmer C. Stewart, Highland Heights,and William S. Foulks, Jr., Chagrin Falls, Ohio, assignors to KewaneeOil Company, Bryn Mawr, Pa.

Original No. 3,532,777, dated Oct. 6, 1970, Ser. No. 574,227, Aug. 22,1966. Application for reissue Oct. 5, 1972, Ser. No. 295,285

Int. Cl. C09k 1/00 US. Cl. 264-21 9 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE This [division] invention comprises pelletsadapted to be made thermoluminescent upon exposure to radiation and theprocess of preparing such pellets by the steps of pressing a powder of athermoluminescable material such as lithium fluoride, calcium fluoride,etc., at a pressure of at least 10,000 p.s.i. while at a temperature inthe range of room temperature to 825 C., said pressure being at least200,000 p.s.i. and having a progressively higher minimum pressure whentemperatures less than 200 C. are used. Such pellets are adapted for usein thermoluminescent dosimeters for measurement of radiation dosages.

This invention relates to pellets adapted for use in a thermoluminescentdosimeter. More specifically, it relates to thermoluminescable pellets,such as thermoluminescable lithium fluoride pellets, etc. suitable forthis purpose. More specifically, it relates to a method of converting ananisotropic or non-uniform thermoluminescable powder, such asactivated-lithium fluoride, activated-calcium fluoride, etc. toisotropic or uniform activated lithium fluoride, etc.

As a precaution against being exposed to more radiation than can besafely tolerated, persons exposed, either intentionally or unavoidably,to ionizing radiation of various types generally carry a device formeasuring the amount of radiation to which they are exposed.

Sometimes this exposure is measured by carrying on the person aphotographic negative which is eventually developed to show the amountof exposure. This method has various well-known disadvantages.Accordingly the use of a thermoluminescable powder offers advantages.One of the most advantageous is activated lithium fluoride. Suchthermoluminescable powders have the ability to absorb radiationequivalent to the amount to which the person is exposed. Since a portionof the energy of this radiation is accumulated, it is testedperiodically to determine the radiation exposure.

The method of testing the accumulated radiation is based on the factthat when the powder is heated it emits light of an intensity inproportion to the amount of radiation accumulated. Instruments have beendeveloped which can measure the amount of light thus emitted andtranslate this information to give a reading in roentgens of radiationto which the material has been exposed.

While other materials can be used for this purpose, it has been foundthat activated lithium fluoride has a number of advantages over theseother materials, most important of which is the fact that lithiumfluoride has a more uniform response to radiation over a wider range ofenergy levels than these other materials such as calcium fluoride,silver activated phosphate glass, and even a photographic negative.

Moreover, activated lithium fluoride more closely resembles human tissuein its response to ionizing radiation.

For this purpose lithium fluoride has an effective atomic number forphoto electric absorption of 8.14, as compared to 7.42 for tissue and7.64 for air. In contrast, glass rod dosimeters have effective atomicnumbers from 15 to 30. calcium fluoride 16.5 and film approximately 40.Moreover, lithium fluoride is inert, insoluble, non-toxic and hasexcellent long-term dose storage properties.

One of the effective methods for preparing the lithium fluoride for thispurpose, is to have one or more activators, such as describedhereinafter, to assist the lithium fluoride in this function. Theactivator must be in very intimate contact with the lithium fluoride andmust be uniformly distributed therethrough. It has been the prac' ticeto add the activator to molten lithium fluoride and then allow thelithium fluoride to crystallize under conditions which promote theformation on one large crystal. In this way the activator becomesdistributed throughout the crystal lattice and is very effective in itsfunction of activating the lithium fluoride in radiation absorption.

Various activators for lithium fluoride and the method of incorporatingthem in the crystal lattice by crystal growth are described and claimedin copending application Ser. No. 322,248 filed by Carl F. Swinehart onNov. 7, 1963.

However, while it would be desirable to cut from the large crystal,small pieces of the activated ltihium fluoride for use in a dosimeter todetermine the amount of radiation exposure, it has been found that theactivator is not uniformly distributed throughout the crystallinelithium fluoride and therefore a piece cut from one part of the crystalwould not be uniform in radiation absorption and resultant lightemission as compared to a piece cut from another portion of the samecrystal. For this reason, it has been found desirable to grind thecrystal to a powder, advantageously to a particle size of less than mesh(Tyler), and to blend the resultant powder with itself until the totalmixture has attained a uniformity with respect to the distribution ofactivator in the lithium fluoride. The activator is still contained inthe crystal lattice of the lithium fluoride, but the powdering andblending operation has converted the lithium fluoride from theanisotropic form found in the large crystal to an isotropic, moreuniform form in the blended powder.

While the powder has been found suitable for use in thermoluminescentdosimeters, there are various disadvantages to its use in this formsince it requires careful weighing to obtain the appropriate standardamount used for test purposes. Moreover, the powder must be used in acontainer, and there is always the risk of spilling or losing some ofthe powder. Furthermore, variations in the spreading of powder in thedosimeter can cause variations in readings, etc. Still furthermore, inthe use of thermoluminescent powders there is a com siderable amount oftribothermoluminescence or noise" which is extraneous to the signalbeing measured.

While the above discussion relates to the preferred lithium fluoridethermoluminescable material, it pertains also to the various otherthermoluminescable materials, such as calcium fluoride, calcium sulfate,etc.

In an attempt to avoid the disadvantage of using such thermoluminescablematerials in powder form, attempts have been made to prepare wafers ofthe thermoluminescable powder suspended in a plastic. This is oftenundesirable since it is impossible to obtain entirely satisfactoryresults or uniformity from wafer to wafer in thermoluminescentproperties usually because of thermal degradation or physical changes.

In accordance with the present invention, it has now been found thatthis powder can be pelletized either by extrusion or by hot pressingunder appropriate temperature and pressure conditions followed bysuitable slicing or cutting operations to produce pellets of standard oruniform size and properties suitable for use as a thermoluminescentdosimeter. Most surprisingly it has been found that [extreme]temperature and pressure conditions generally used in these pelletizingoperations do not destroy the thermoluminescent properties of thesematerials and in some cases improves the signal to noise" ratio.

Moreover, it has been found that with appropriate conditions of pressureand temperature it is possible to start with individual powders oflithium fluoride or calcium fluoride, etc. and their respectiveactivators in appropriate particle size, and after uniformly blendingthe mixture of powders, to pelletize the mixture according to theconditions described herein to produce a pellet having the lithiumfluoride and the activator in such intimate contact as to effectthermoluminescable properties previously attainable only by crystalgrowth in a molten salt-activator mixture. Apparently the temperatureand pressure conditions used either to extrude or to press the mixtureincorporates the activator into sufliciently intimate contact with orwithin the lithium fluoride crystal lattice to give thethermoluminescable properties. This latter method has the obviousadvantage of eliminating the melting and crystal forming steps.

In either the compressing or extruding operation used in producing thepellets of this invention, it is desirable to use powders of lithiumfluoride and of the activator having a particle size of less than 0.1inch in dimension, advantageously 80-200 mesh (Tyler) or smaller. If thepowder is one formed by crushing or grinding a lithium fluoride crystalin which the activator has already been added to molten lithium fluorideprior to crystallization, as described above, then a separate activatorpowder will not be added.

The powder is either compressed or extruded into a bar. and subsequentlycut into pellets of the desired size. Advantageously the extruded orpressed product has a square cross-section, although other cross-sectionshapes, such as rectangular, diamond or circular also can be used. Theeither pressed or extruded bar [or extrusion], or even the pelletitself, is advantageously polished to give a smooth surface. It is alsoadvantageous to have the corners of the square cross-section, etc.slightly rounded, particularly with extrusion products since thefrictional drag during extrusion sometimes causes roughness or openingsalong the sharp edges.

In the polishing operation referred to above, this is performeduniformly on each red, ribbon, or pellet so as to equalize the smallamount of material removed in the polishing operation. In any case,there is not a sufiicient loss of material as to create suflicientvariations of weight that might alter or make non-uniform thethermoluminescent properties when eventually used in the dosimeter.

Moreover the pellets can be produced of a standard size to be used in ameasuring instrument calibrated for that particular size. A particularlyadvantageous size is one having a square cross-section, preferably withrounded corners, the dimension of each size of the square being0.0600.06S inch and the length being 0.5 cm. Another convenient size,with approriate change in calibration of the instrument is a pellethaving the same cross-section as above and a length of one cm.

In either the pressing or extruding operation, a dry, inert atmosphereis advantageously used to avoid any bad effect the moisture in air mayhave on the thermoluminescable properties at the raised temperaturesgenerally used. Consequently, the pressing mold or extruder is generallyeither evacuated or swept with an inert gas, such as nitrogen, argon,etc. and after the powder has been added, an atmosphere of inert gas ismaintained with the temperature being appropriately raised.

Moreover, the product is advantageously extruded into an inertatmosphere which is maintained until the temperature of the material hasbeen lowered to room temperature. Likewise in the pressing operation,the material in the die advantageously is kept in the inert atmosphereuntil the temperature of the material has reached room temperature. Aspreviously stated. the [extruded] [or pressed] product, whether extrudedor pressed, is cut to desired dimensions. If needed or desired, theproduct is polished before or after the cutting to remove any surfacediscontinuities.

This process produces a large number of transparent or translucentpellets of standard size, all normalized and made isotropic startingwith an anisotropic activited lithium fluoride, or a mixture of lithiumfluoride and an activator, or other activated thermoluminescent powderas described above.

In the drawings, FIG. 1 is a plan view of an extruder taken at line 1-1in FIG. 2.

FIG. 2 is a front elevational cross-sectional view of the same extrudertaken at line 2-2 of FIG. 1.

FIG. 3 is a plan view of direct compression equipment comprising a ringcontaining compressed powder for direct compression taken at line 3-3 ofthe view shown in FIG. 4.

FIG. 4 is a front elevational view of the compressing equipment shown inFIG. 3 and taken at line 44 of that view.

FIG. 5 shows two curves plotted to represent the minimum pressures thatcan be used at various pressures, one curve representing values forextrusion and the other for direct pressing. The pressures are plottedon logarithmic scale.

In the views of FIGS. 1 and 2, the extruder main body 1 has a well 2filled with powder. The base of the well is an insert plug 8 which hasan orifice 8'. Plug 8 is designed to fit tightly into the bottom of well2 but can be removed for cleaning purposes by raising the extruder mainbody 1 and forcing piston 6 all the way through well 2. The plug 8 canalso be removed for replacement by a plug having a different sizeorifice.

This extruder is surrounded by furnace 4 which has the heating elements4' adjacent to the extruder main body 1 and the exterior of the furnacecovered by insulation 4". Thermocouple 5 with lead wires 5" is affixedto the outer surface of extruder main body 1. After the powder has beenheated for a suflicient period for it to reach the desired temperature,the piston 6 is forced by means of plunger 7 of the hydraulic press (notshown) to the interior of well 2, thereby extruding the contents of thewell through the orifice 8' to give the extrudate 9. The die and furnaceare supported by table 11 which has an opening under the die for passageof this extrudate. This extrudate enters box 10 which in the uppersection is made of transparent plastic 10' and the lower portion iscardboard 10" lined on the inside with aluminum foil so that theextrudate will touch only metal. A nitrogen source (not shown) maintainsa nitrogen atmosphere in the interior of box 10. To facilitatemanipulation and cutting of the extrudate, a door (not shown) isprovided.

FIGS. 3 and 4 show equipment used in a preferred method of makingcompressed discs for cutting into pellets according to this invention.In FIG. 3, the ring 13 having either a square or rectangular annularcross-section as shown in FIG. 4 has its interior filled with compactedpowder. This ring is preferably of stainless steel or steel. The ringand its contents are supported on support block 12. After the ring andits contents are heated to the desired temperature by heating element14, as determined by a thermocouple which is not shown, the plunger 7 isapplied to ring 13 and the powder 3 with sufficient force to compressthe steel of the ring. By simultaneously compressing the ring and thepowder, a more uniform pressure is exerted throughout the powdercontained in the ring. The ring is found to flow outwardly in such amanner that both the inner diameter and the outer diameter of the ringare increased by the compression.

It is believed that the pressure exerted on the powder is transmitted tothe inner wall of the ring, thereby causing it to expand and likewisecausing the outer diameter to expand. In this way the steel ring iscompressed to about /3 or V2 of the original height of the ring. As aresult of the increase of the inner diameter of the ring and theaccompanying increase in area within the ring, the pressure decreasesduring the pressing operation when a constant force is appliedthroughout by the plunger 7. The resultant disc of compressed powder canbe pushed out from the interior of the ring and recovered easily forsubsequent treatment. This disc is polished and then cut into desiredpellet size. For direct compression, this method is preferred since themethod of recovering the disc is simplified.

It is also possible to make a compression disc using the equipment ofFIG. 2 but using, in place of the die 8 which has an orifice therein forextrusion purposes, a plate of approximately the same thickness as die 8but having no orifice therein.

After the desired degree of compression has been exerted on the powderin well 2, using only suflicient powder to give the desired thickness ofcompressed disc, the pressure is released so that the main body 1 can beraised from the supporting table 11 and supported in such a manner thatthere is no barrier below disc 8 which will prevent its being pushedfrom well 2 when the pressure is reapplied at the top of the compactedpowder above the die 8. In this way, both the die 8 and the compactedpowder disc are pushed from well 2.

The particular temperature and pressure used in pressing a productaccording to this invention, either in extrusion or in direct pressing,can vary over a wide range, with the selection of the temperature andthe pressure being interdependent upon each other. For example, in thelower portion of the temperature range cited herein, it is desirable touse pressures in the higher portion of the cited pressure range. Thus,if the room temperature is to be used in an extrusion, this isadvantageously done with a pressure of about 1.5 million p.s.i. Ifhigher temperatures are selected, the pressure for obtaining practicalrates either of extrusion, or of compression in the case of pressedproducts will be reduced accordingly.

Generally, however, the temperature range is from room temperature to825 C. for either compressing or extruding. The pressure range forextrusion is advantageously 20,000 p.s.i. up to 1.5 million p.s.i. ormore depending on the temperature and desired rate of extrusion. Incompressing operations, pressures in the range of 10,000 up to 1.5million p.s.i. or more can be used. For example at room temperature apressure of at least 400,- 000 p.s.i. should be used, and at 825 C.pressures as low as 10,000 p.s.i. can be used. Generally, extrusionrequires greater pressures than for direct compression of product atequivalent temperatures. Furthermore, with smaller extrusion orifices,it is desirable to use greater pressures.

In commercial operations using thermoluminescable lithium fluoride, itis preferable to use a temperature of about 700 C. for extrusion, and apressure of 50,000 65,000 p.s.i. The higher temperatures allow fasterextrusion rates and better transparency in the product. For directcompression, a practical commercial temperature is 250-300 C. withpressure about 300,000 p.s.i.

For thermoluminescabie calcium fluoride, a very practical set ofconditions for direct compressing to transparent products is 600 C. and100,000 p.s.i. In extruding thermoluminescable calcium fluoride, higherpressures are advantageous than for lithium fluoride. Similar conditionsare used for thermoluminescable calcium sulfate pressings and extrusionsasused for thermoluminescable calcium fluoride.

The following table illustrates the variations in the temperatures andpressures for extrusion of activated lithium fluoride:

Pri-ssuris suilnl tie for Minimum extrusion pressure, rate, p.s.i.p.s.i.

When using thermoluminescent powders as described herein, there is aconsiderable amount of tribothermylluminescence, or noise due to falseradiation. The *noise" generally refers to the effects produced otherthan the desired signal or type of radiation or light emission which itis pressed, the pressing eflects an improvement truding of the powdermay reduce the thermoluminescity a small degree as compared to that ofthe powder from which it is pressed, the pressieng effects animprovement in the ratio of thermoluminescence to noise. This means thata considerable amount of the tribothermylluminescence or noise iseliminated so that a truer signal can be read in the testing equipment.

The pellets of this invention have the advantage of being easier tohandle and use than a powder. The pellets are also transparent or atleast translucent. Another advantage is the fact that the pelletsproduce a high signal to noise ratio as shown in the electronic circuitmeasuring the [like] light quantity. This is a considerable advantagesince it permits more accurate measurement and permits measurement atlower energy levels.

The invention is best illustrated by the following examples. Theseexamples are given merely for illustrative purposes and it is intendedthat neither the scope of the invention nor the method in which it maybe practiced is to be limited in any manner by these examples. Unlessspecifically provided otherwise, parts and percentages are given byweight.

EXAMPLE I An extrusion of thermoluminescable powder is performed usingthe equipment shown in FIGS. 1 and 2 with an activated lithium fluorideas prepared in Example I of the above-mentioned copending applicationand containing 400 parts of magnesium fluoride, 200 parts of lithiumcryolite and parts of lithium fluotitanate per million parts of lithiumfluoride. The powder has a particle size of -200 mesh (Tyler). Prior tothe insertion of the powder into the extruder, the space in the well isswept free of moist air by feeding a stream of dry nitrogen into thebottom of the well and maintaining a nitrogen atmosphere during andafter the addition of the powder. The well has a diameter of [0775"]0.75" and a depth of 6" above the plug or die 8. The orifice has across-section 0.06 inch square. The well is filled with powder which iscompacted to approximately of the height of the well. Then the piston islowered into position to rest upon the compacted powder. Heat is appliedto the extruder and the extruder kept at a temperature of 550 C..according to the reading of the thermocouple aflixed to the main body ofthe extruder, for a period of two hours to insure that the powder hasattained a uniform temperature. Force is then applied to the plunger ofthe hydraulic press such that a pressure of 100.000 p.s.i. is aplied atthe surface of [to] the powder. In 15 minutes an extrudate length ofabout 3 inches is obtained. The pressure is raised to 133,000 p.s.i.which increases the rate of extrusion to about Va inch per minute. Inabout 1.5 hours a length of about 22 inches is extruded. Various samplesof about /1 inch long are cut and polished on two parallel sides. Theseare clear and transparent and when placed upon printed material it ispossible to read the print through the material. The cross-section isapproximately 0.06 inch square and lengths of 0.5 cm. are cut fortesting. Each sample is exposed to 100 rads of gamma radiation having anenergy level of about 300 Kev. The thermoluminescence of each sample sotreated is measured using a reading system analogous to the ResearchReader" described in an article by G. N. Kenny et al., Rev. Sci. Ins.,34, No. 7,769 (1963). In each case, the thermoluminescence is easilyread and found to be approximately identical, which shows that thethermoluminescence is a true indication of the amount of radiationreceived. Similar results are obtained when the radiation is X-ray, oralpha or beta-rays, and when the radiation dosage is varied.

EXAMPLE II The procedure of Example I is repeated using a temperature of700 C. and a pitson pressure of 60,000 p.s.i. Excellent results areobtained upon radiation and testing of luminescence of the resultantsamples as in Example I. Satisfactory results are also obtained with thefollowing variations in temperature and pressures:

650 C. and 120,000 p.s.i.

825 C. and 20,000 p.s.i.

250 C. and 550,000 p.s.i.

EXAMPLE III The procedure of Example I is repeated using an orificehaving a cross-section of 0.09 inch square.

EXAMPLE IV The procedure of Example I is repeated a number of timesusing in place of the activated lithium fluoride of that example anumber of activated lithium fluorides prepared according to theprocedure of the said copending application and containing per millionparts of lithium fluoride, the following activator compositions:

(a) 400 parts calcium fluoride, 200 parts lithium cryolite,

and 55 parts lithium fluotitanate;

(b) 400 parts barium fluoride, 200 parts lithium cryolite,

and 55 parts of lithium fluotitanate;

(c) 400 parts magnesium fluoride, 200 parts lithium cryolite and 50parts europium fluoride;

(d) 400 parts magnesium fluoride, 200 parts lithium cryolite, 55 partslithium fluotitanate and 25 parts of europium fluoride;

(e) 40 parts magnesium fluoride, 20 parts lithium cryolite and 42 partsof lithium fluotitanate;

(f) 40 parts barium fluoride, 60 parts lithium cryolite and 50 partseuropium fluoride;

(h) 40 parts calcium fluoride, 50 parts lithium cryolite and 50 partslithium fluotitanate;

(i) 200 parts magnesium fluoride, 100 parts lithium cryolite and 60parts lithium fluotitanate;

(j) 400 parts magnesium fluoride, 30 parts lithium cryolite and 55 partslithium fluotitanate;

(k) 300 barium fluoride, 20 parts lithium cryolite and 42 parts lithiumfluotitanate; and

(l) 40 parts magnesium fluoride, 80 parts barium fluoride,

200 parts lithium cryolite and 42 parts lithium fluotitanate.

In each case satisfactory results are obtained upon testing theresultant pellets with radiation and thermoluminescence as described inExample 1.

EXAMPLE V The procedure of Example I is repeated using instead of thegrown crystal of lithium fluoride and the various activators described,mixtures of the lithium fluoride and the activators as they are usedprior to melting for subsequent crystal growth. Instead these activatorsare added to the lithium powder as such and placed in the extruder well.The resultant pellets have a substantial amount of thermoluminescentproperties when radiated and tested according to the procedure ofExample 1. Thermoluminescent pellets are also obtained when the lithiumfluoride is mixed with the various activators described in Example IV,but without using the crystal growth technique. After radiation, asubstantial amount of thermoluminescence is found in each case whentested as in Example I.

EXAMPLE VI The procedures of Examples IIII are repeated, using anactivated-calcium fluoride powder as prepared in Atomic Energy Review,"p. 23 vol. 3 (i966), pp. 84-86, having approximately 3 mole percent ofmanganous fluoride as the activator.

Upon cutting and radiating as described in Example I the resultantpellets exhibit satisfactory thermoluminescent properties.

EXAMPLE VII The procedures of Examples I-III are repeated using anactivated calcium sulfate powder, prepared as described in Atomic EnergyReview, vol. 3 (1966), pp. 92-93, containing approximately 1 percentmanganous sulfate. Similar results are obtained as in Example VI.

EXAMPLE VIII A compressed disc is made using the activated-lithiumfluoride powder of Example I and equipment as shown in FIGS. 3 and 4. Astainless steel ring is used having an inner diameter of 0.53 inch, anouter diameter of 0.63 inch, and the annular section having arectangular crosssection 0.25 inch high and 0.05 inch wide. The powderis cold pressed in a nitrogen atmosphere at a pressure of 100,000 p.s.i.to form a disc inch high which will just fit into the interior of thering. The ram of the hydraulic press is brought to rest on the top ofthe ring and powder. With a thermocouple attached to the exterior of thering, heat is applied and the temperature maintained for 1 hr. 40minutes at 220 C. to make sure that the mass of powder is uniformlyheated. Then a pressure of 320,000 p.s.i. is applied for 2 minutes tothe ring and powder at which time the ring has been reduced to athickness of 0.125 or approximately V: the original height. Then thecompressed ring and pressed powder disc are removed from the equipmentand the powder disc separated from the ring. When the disc is removedfrom the ring, it has clear top and bottom surfaces and is transparent.It is cut into a number of pieces 2 mm. x 2 mm. x 10 mm. Beforeradiation, the respective pieces are heat-treated at 400 C. for mm. and20 hours at 80 C. The various samples are simultaneously raidated to adose of rads of X-ray. The thermoluminescence is tested as in Example Iand found to show excellent thermoluminescence properties andreproducible results as shown by the proximity of reading valuesobtained from the respective samples.

EXAMPLE IX The procedure of Example VIII is repeated with satisfactoryresults using the following variations in temperatures and pressures inthe pressing operation:

(a) room temperature and 700,000 p.s.i.;

(b) 300 C. for one hour, hot-pressed one hour at 225,000

p.s.i., and cooled one hour at 50 C.; and

(c) 600 C. and 46,000 p.s.i.

EXAMPLE X The procedure of Example VIII is repeated a number of timesusing respectively the various activated-lithium fluorides of ExampleIV. Satisfactory results in thermoluminescence are obtained in each caseupon testing as in Example I.

EXAMPLE XI The procedure of Example VIII is repeated with satis factoryresult at 650 C. and 325,000 p.s.i. when activated calcium fluoride ofthe type used in Example VI is used as the powder.

9 EXAMPLE XII The procedure of Example VIII is repeated withsatisfactory resuls, except that activated calcium sulfate of the typeused in Example VII is used as the powder.

EXAMPLE XIII The procedure of Example VIII is repeated at 650 C. and325,000 p.s.i. using, instead of the powder of those examples, a mixtureof calcium fluoride and activator as used in Example VI having particlesizes in the range of 80-200 mesh. Satisfactory thermoluminescenceproperties are obtained when tested as in Example I.

As indicated above, the extruded or compressed product is preferablyclear and transparent, particularly after the surfaces are polished.This is desirable for maximum transmission of light duringthermoluminescence. However, even with translucent products there issufficient light transmission to make the products suitable for thepurposes of this invention.

With regard to the extrusion operations described herein, the size ofthe orifice in the extrusion die is not critical so long as it permits asufl'icient buildup of the pressuge required to give the necessarycompaction for producing transparent or translucent product. Thisparticular pressure will vary according to various factors such astemperature, particle size, type of powder, etc. as discussed above.Generally, however, it has been found advantageous to have an orificehaving its dimensions in the range of 0.04 to 0.120 inch, preferablyabout 0.06 inch.

While certain features ofthis invention have been described in detailwith respect to various embodiments thereof, it will, of course, beapparent that other modifications may be made within the spirit andscope of this invention and it is not intended to limit the invention tothe exact details shown above except insofar as they are defined in thefollowing claims.

The invention claimed is:

1. A process for preparing pellets adapted to be made thermoluminescentupon exposure to radiation comprising the step either of pressing orextruding a powder of a thermoluminescable material at a pressure of atleast 10,000 p.s.i. while at a temperature of at least room temperatureand no higher than 825 C., said pressure being [at least 400,000 p.s.i.when said temperature is in the Cir range of room temperature to 100 C.,at least 300,000 p.s.i. when said temperature is in the range of 100 to200 C., and at least 200,000 p.s.i. when in the range of ZOO-300 C.]less than 1.5 million p.s.i., greater pressures being required at lowertemperatures.

2. A process of claim 1 in which said powder is predominantly of aparticle size smaller than mesh Tyler.

3. A process of claim 1 in which said powder is predominantly of aparticle size in the range of 80 to 200 mesh Tyler.

4. A process of claim 1 in which said pressure and said temperature areeffected on said powder during extrusion of said powder, and theresultant extrudate is cut into desired pellet size, said temperaturebeing at least 200 C. [and said pressure being at least 100,000 p.s.i.]

5. A process of claim 1 in which said powder is thermoluminescablelithium fluoride.

6. A process of claim 5 in which said temperature is about 700 C. [andsaid pressure is about 50,00065,000 p.s.i.]

7. A process of claim 1 in which said powder is thermoluminescablecalcium fluoride.

8. A process of claim 1 in which said powder is a mixture of lithiumfluoride and an activator for making said lithium fluoridethermoluminescable.

9. A process of claim 1 in which said powder is a mixture of calciumfluoride and an activator for making said calcium fluoridethermoluminescable.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,994,109 8/1961 Thomas 264-1 11 3,141,973 7/1964Heins 252-301.4

3,282,855 11/1966 Palmer 252301.4

3,312,759 4/1967 Letter 264-332 US. Cl. X.R.

THEODORE MORRIS, Primary Examiner

1. A PROCESS FOR PREPARING PELLETS ADAPTED TO BE MADE THERMOLUMINESCENTUPON ESXPOSURE TO RADIATION COMPRISING THE STEP EITHER OF PRESSING OREXTRUDING A POWDER OF A THERMOLUMINESCABLE MATERIAL AT A PRESSURE OF ATLEAST 10,000 P.S.I. WHEN AT A TEMPERATURE OF AT LEAST ROOM TEMPERATUREAND NO HIGHER THAN 825*C., SAID PRESSURE BEING (AT LEAST 400,000 P.S.I.WHEN SAID TEMPERATURE IS IN THE RANGE OF ROOM TEMPEATURE TO 100*C., ATLEAST 300,000 P.S.I. WHEN SAID TEMPERATURE IS IN THE RANGE OF 100* TO200*C., AND AT LEAST 200,000 P.S.I. WHEN IN THE RANGE OF 200-300*C.)LESS THAN 1.5 MILLION P.S.I., GREATER PRESSURES BEING REQUIRED AT LOWERTEMPERATURES.